Method of making a heat exchanger



Feb. 23, 1954 'w. s. NORMAN ET AL METHOD OF MAKING A HEAT EXCHANGER 3 Shets-Sheet 1 Filed June 1 1951 ruveu'ra R 5 Feb. 23, 1954 w. s. NORMAN ETAL 2,670,311

METHOD OF MAKING A HEAT EXCHANGER Filed June 1. 1951 3 Sheets-Sheet 2 INve/vTaRS Feb. 23, 1954 w, s, NORMAN ETAL- I 2,670,311

METHOD OF MAKING A HEAT EXCHANGER Filed June 1, 1951 3 Sheets-Sheet 3 I v l l I 1 l I I Iuveuroks 6 715W. 1R, MMM

Patented Feb. 23, 1954 METHOD OF MAKING A HEAT EXCHANGER William Stephens Norman, Chester, and Charles Henry Victor Sawyer, Hayes, England, assignors to C. D. Patents Limited, London, England,

a British company Application June 1, 1951, Serial No. 229,306

Claims priority, application Great Britain February 2, 1951 8 Claims.

This invention relates to improvements in heat exchangers in which the heat exchanger body is constructed or substantially constructed of carbon.

United States Patent specification Serial No. 120,480 describes a heat exchanger body composed of carbon having first rows of parallel passages for one fluid and adjacent second rows of parallel passages for another fluid, the direction of the first rows crossing the direction of the second rows. It is an important feature of that heat exchanger as described with reference to the drawings that the body is built up of contacting carbon plates and the invention includes a heat exchanger body built up of contacting carbon plates each plate having on one side a first set of equally-spaced parallel grooves and on the other side a second set of equally-spaced parallel grooves lying in a direction at right-angles to the direction of the first set, the alternate contiguous plates being so arranged that the first set of grooves lies in register with the first set 3f grooves in a contiguous plate and the second set of grooves lies in register with the second set of grooves in a contiguous plate so as to form first and second sets of passages at right-angles to one another.

Again the invention described in United States specification serial No. 120,480 includes a heat exchanger body in which the assembled plates are impregnated and bonded together by evacuation, immersion in an impregnating and bonding liquid subjecting the assembly to hydraulic pressure, removing excess liquid and heating the assembly to complete the bonding.

The present invention belongs to the same type and consists in developments of the actual process of production of the heat exchanger embodying not only the heat exchanger body composed of carbon but also the clamping plates, the headers and the header cover plates.

According to the present invention the assembly of carbon plates forming the heat exchanger body is maintained in a state of compression between clamping plates of rigid construction (for example of cast iron or steel) drawn together by bolts at each corner, as is shown in the drawings appended. The special feature of this form of construction is that the bolts joining the clamping plates are tightened so as to exert a predetermined compressive stress on the carbon assembly, this compression being sufiicie'nt to overcome the tensile stress which would otherwise be set up in the carbon by the pressure of the fluid in the passages, and to maintain the Qefiifllted 2 joints between the plates tight and free from leaks. As is well known carbon has a much higher strength in compression than in tension (typical figures for the compressive and tensile strengths being respectively 20,000 and 4,000 lbs/sq. in.) and by exerting an external compression load on the heat exchanger body the strength characteristics of the carbon are used to their best advantage, and the heat exchanger can withstand much higher fluid pressures than would be the case if the bolts joining the clamping plates were merely tightened sufiiciently to hold the heat exchanger body in position.

According to the present invention the bolts joining the clamping plates holding the carbon plates together are tightened to a predetermined pressure prior to the curing of the impregnating and bonding agent, the pressure being sufficient to withstand the pressure which will be exerted by the fluid within the heat exchanger body and also being sufficient to make the cemented joints proof against leaks.

It is a feature of the present invention that even during the impregnation of the plates of carbon, the bolts of the clamping plates holding the carbon plates together are tightened (e. g. hand-tight) so that the constituent parts of the heat exchanger body and the clamping plates are securely held in their final positions.

Experience has shown that a carefully controlled technique of assembly, impregnation and curing must be followed to produce a carbon heat exchanger body having the desired characteristics of high heat transfer, ability to withstand fluid pressure and freedom from leaks when such a body is constructed by the assembly, cementing and impregnation of grooved carbon plates in the manner described in the United States Patent specification Serial No. 120,480 and referred to above. The technique of manufacture will now bedescribed, but it should be understood that minor variations may be called for according to the type and size of the heat exchanger, and in particular according to the type of cementing and impregnating resin employed. This latter must be selected in accordance with the nature of the fluids to be handled by the heat exchanger, and the temperature and other conditions of service.

This invention includes a method in which carbon plates are formed by known processes of pressing and firing mixtures of carbon and/or carbonaceous materials such as coal, pitch or tar,

the carbon plates machined or ground to the requisite size, groves formed in the plates by machining or grinding, the plates assembled with the grooves in register as described above, and the assembly clamped between clamping plates of rigid construction, for example of cast iron or steel. The grooves may be of semi-circular cross section, in which case the passage formed by the grooves between contiguous plates will be circular, but the invention is not limited in this respect and for certain cases it is advantageous to employ deep narrow grooves of substantially rectangular cross-section. At this stage of the assembly the bolts joining the clamping plates are not tightened fully, and they may be only hand tight.

This invention includes a method of making a heat exchanger in which the assembled carbon plates with their clamping plates are introduced into a container which is then closed and is subjected to a vacuum (about 3 millimetres of mercury absolute) for about an hour, the resin (such as cashew nut shell resin or such as fur- !uryl alcohol/sulphuric acid resin) at about 70 (3., allowed to enter the container from the lower part thereof to immerse the body in the resin; the container is subjected to air pressure at about 100 lbs. per square .inch for a period of about two hoursthe pressureiis released gradually and the body is allowed to drain, after-which the bolts joining the clamping plates are tightened up equally each to a pressure between the contacting faces of the carbon plates between 400 and 500 lbs. per square inch.

This invention further includes a method of making a heat exchanger .in which the curing of the cementing and impregnating resin cashew nut shell resin is effected by placing the impregnated body .in an oven at abcut:80 C. for "about 24 hours, at intervals of an hour or more during that period removing the body from the oven and allowing it to cool to permit any exuded resin to be reabsorbed, then continuing to heat the body for another period of about 24 hours at about 120 C. and finally heating the body fora further period-of 24 hours at about 150 C., after which thebody is allowed to cool.

This invention further includes a method of testing an impregnated and cured body of a heat exchangeras described above in which anopposed pair of headers is applied to the block of carbon plates and the header outlet is closed and the header inlet is subjected to substantial air pressure (about 50 lbs. per square inch), the body is immersed ina bath of-water and the blockisexamined for any'trace of leak, and in the case of a leak the header are removed and the block is re-impregnated and again subjected'to the curing operation.

To enable the clamping plates, headers and header cover plates to be standardized so that one header form (c. g. the inlet andoutlet header form) may serve for any side of the heat exchanger body and one form of clamping plate will serve for top or bottom clamping plate no matter which side receives the inlet and outlet header, the number of holes running in one direction is the same as the number ofholes running in the direction at right angles thereto (say 12 .rows or 16 rows in the case where one fluid may make 4 passes throughthe block) :and the bolts securing the headers lie one setinside the normal set and the other set outside the normal set.

The natureof this invention and the manner in which it is performed willbe appreciated from the following description of, an example, ref-- '4 erence being made to the accompanying drawings in which:

Figure 1 is a plan,

Figure 2is asection on the line X.X of Figure 1, and

"gum 3 is a section on the line Y-Y of Figure 1.

The heat exchanger body proper is built up from a series of plates l2 of carbon which has been converted partly or wholly to the raphitic form. According to the preferred process each plate is moulded from very finely pulverised coal with or withouta small proportion of very finely pulverised coke or graphite and/or finely divided sulphur which after moulding to the required dimensions (allowing for subsequent shrinkage, e. g. 11% linear shrinkage) is embedded in coke fliers and fired under non-oxidising conditions to a temperature of 800 to 1000 C. at such a rate of temperature rise as to avoid intumescence as described in United States Patent specification No. 2,461,365 or 2,493,383. The plates may thereafter be heated to a temperature of 2500 C. in an electric furnace. Each plate is ground flat on eac'hside in a grinding'machine and each edge of each plate is ground straight and to theccrrect dimensions.

The conditions at this stageof the process are controlled in such a way that the carbon is not fully converted to graphite but into an intermediate form which has a high thermal conductivity (about 75 of that of natural graphite) but a higher strength and hardness than fully graphitised carbon.

Grooves are formed in the carbon plates by grinding, using a machine with a rotary grinding wheel the peripheral edge of which conforms to the shape of the groove required in the carbon plate. Thus for example when semi-cylindrical grooves are to be formed a grinding wheel having an edge of semi-circular section is employed. Each plate is provided on one side with afirst set of equally-spaced parallel semi-cylindrical grooves 13 and on the other side with a second set of equally-spaced parallel semi-cylindrical grooves 14 lying in a direction at right angles to the direction of the first set i3, the alternatecontiguous plates I2 being so arranged that the first sets of grooves 13 lie in register with the first sets of grooves I3 in a contiguous plate 12 and the second sets of grooves 15 lie in register with the second sets of grooves I 4 in a contiguous plate l2 so as to form first and second sets of cylindrical passages at right angles to one another.

At the top and bottom of the pile of plates 12 are somewhat thicker carbon plates 15 treated like the plates 12 but having semi-cylindrical grooves it only on the inner face so as to complete the top and bottom sets of cylindricalpassages.

The plates are assembled as described above and held between clamping plates H with bolts l8 and nuts i9 passing through lugs 30. At this stage of the manufacture the nuts and bolts are tightened lightly sumcient to hold the plates in position but not imposing any appreciable compression. At this stage the exposed faces of the block are machined perfectly flat. The assembly is cleaned to remove any carbon dust and dried out by heating in an oven at about C. for a period of about 3 hours.

The impregnation and bonding agent may comprise any suitable resinous material which the unpolymerised or partly polymerised state is sufriciently fluid to permeate the porous voids in the carbon, and which can be polymerised by the action of heat and/or chemical reagents to form a hard cement after the impregnation. The description of the impregnation process which follows refers to the use of a natural resin known as cashew nut shell resin, but it should be understood that other natural or synthetic resins may be employed, and that the use of these resins may necessitate minor modifications to the process.

The cashew nut shell resin is mixed with about of diethyl sulphate as a polymerisation catalyst, and immediately prior to use it is heated to C. to reduce its viscosity and facilitate the impregnation. The impregnation is efiected in a container which has an inlet and outlet conduit at the bottom for the resin and a coupling conduit or conduits at the top for connection to vacuum pump or to a supply of compressed air. The block is taken out of the oven and allowed to cool to about C. and then lowered into the container which is closed. Vacuum is applied (say to 3 mm. of mercury absolute for 1 hour). Then the liquid resin is allowed to enter till the block is completely immersed in the resin. Air pressure at about lbs. per square inch is applied for two hours and is then slowly released over a period of about half an hour. The block is then extracted and supported above the container with sets of holes inclined to the horizontal so as to drain out the resin from the holes which are then cleaned out with rods.

The next step (which is of great importance) is that the nuts E9 on the bolts i8 of the clamping plates I! are tightened to a critical high pressure (e. g. 400 to 500 lbs. per square inch) and the degree of tightness is determined to ensure that the block as a foraminous structure shall withstand the internal fluid pressures to which the block may be subjected and to ensure that the cemented joints between the contacting faces of the carbon plates are proof against leaks.

The next step in the process of manufacture consists in the curing or polymerisation of the impregnating and bonding agent, cashew nut shell resin. This is carried out by heating in three stages of 24 hours each. In the first stage the oven is at 80 0.; in the second stage C. and in the third stage at C.

When the impregnated and cleaned block is first introduced into the oven there is a tendency for the liquid resin to exude from the pores of the carbon and from the joints or holes and therefore after the block has been in the oven for a short time (say one hour) to enable the block to get heated up, it is removed from the oven and allowed to cool to allow the resin which has exuded to be reabsorbed. This procedure may be repeated several times, in fact until exudation no longer takes place; then the heating schedule proceeds as above indicated.

In another embodiment of this invention the impregnating and bonding agent is a resin resulting from the reaction between a furfural and sulphuric acid mix at 100 C. for 20 minutes'to carry out the reaction.

(6) Remove source of heat, add 0.16% b weight of caustic soda in normal solution to neutralise the acid and allow to cool.

(1) Distil at 100 mm. pressure absolute up to 88 C. to remove water of condensation.

(g) Acidify with a further quantity between 0.1% and 0.2% of normal aqueous solution of sulphuric acid.

The resin of the desired viscosity is now ready for impregnation.

The heat exchanger is heated at 100 C. for 2 hours to remove any excess water in the material. The block is then placed in a closeable vessel and vacuum applied. After complete evacuation the resin is drawn into the vessel to completely cover the block and air pressure applied at 100 pounds per square inch for 2 hours.

The block is then removed from the vessel, excess resin allowed to drain, and the block allowed to stand for a further period of about 2 hours during which the surface resin is absorbed into the material. The nuts it on the bolts :8 of the clamping plates H are tightened to a critical high pressure between the contacting faces of the carbon plates amounting to :between 400 and 500 pounds per square inch to ensure that the cemented joints between the carbon plates are proof against leaks. The block is then heated at 80 C. for 9 hours, and at 150 C. fora further three hours.

Irrespective of the type of resin used, it is desirable to test the block for impermeability, and if leaks are apparent, it is necessary to re-impregnate.

For example, to test the-heat exchanger body for the possibility of a leak, one pair of headers is fitted and bolted up and the header outlet is closed and the header inlet is subjected to substantial air pressure (e. g. 50 pounds per square inch), the body is immersed in water and is examined for any trace of air leak and in the event of a leak appearing the re-impregnation is efifected by leaving the headers bolted on, filling the headers and block with resin (on the side where the leak occurred), applying air pressure at about 50 pounds per square inch for about an hour. The headers are then removed and the normal procedure of draining and curing completed. Alternatively the normal process of impregnation and curing may be repeated.

The arrangement of the headers for the supply and discharge of the fluids to and from the passages of the heat exchanger will now be described with reference to Figures 2 and 3. A particular feature of this invention is the design of the headers in such a way that they are interchangeable and may be fitted to eitherset of fiuid passages. The headers may be made of any suitable material, and it is convenient to employ cast iron headers for non-corrosive fluids and headers of cast iron with a corrosion resistant lining(for example a lining of natural or synthetic rubber) or headers fabricated of carbon for corrosive fluids. The headers carry pipe connections for the entry and discharge of the fluids, and they maybe fitted with baflles to cause the fluids to traverse the heat exchanger body two or more times, the arrangement shown in the figures being such as to cause the fluids to pass four times through the exchanger.

The Figures 2 and 3 show two different sections of a heat exchanger having one pair of headers of cast iron (Figure 2) and a second pair of headers. of rubber-lined cast, iron, (Figure. 3). This is the arrangement used when, it is; desired. to. eilect the transfer of heat between a noncorrosive and a. corrosive fluid.

Referring to. Figure 2-; the passaee orm d y the. grooves I4: (and I6) open at the right hand side into the cast. iron header 20;, and at the; left hand side into the header 2.64.. The header 20 has an inlet passage 21 fitted with a steel inlet coupling pipe 24, a reversing passage, 22,, and an outlet passage 23 fitted with a steel outlet pipe 25. The header 26 has an upper reversin passage. 2-7 and alower reversing passage. 28. These headers have. lugs I20: at the corners secured by bolts. I2! having nuts I22. At the. contacting lines of the heat exchanger body and the headers 20. and 26 aregaskets 29 (conveniently of rubber) let. into'grooves on the inner faces of the headers.

Similarly referring to Figure 3- the passages formed by grooves. 13 (and I16) open into a cast iron header 32 with a rubber lining 3| on the right. side of a. header 42 with a. rubber linin M on the left hand side. The header 32 has an inlet passage 36 and rubber lined inlet pipe 39, a. reversing passage 31-, and an outlet passage 38 with a rubber lined outlet pipe 40.. The header 42- has reversing passages 61 and 62.

The headers are held in position bybolts run ning across the top and bottom of the heat exchanger body and passing through holes in the clamping plates H. Headers 20 and 25 are held by bolts 12! and nuts [22,, the bolts passing through lugs I20. at the corners of the headers. Similarly headers 32 and 42 are held by bolts 34 and nuts 35, the bolts passing through lugs 33..

The pairs of headers are interchangeable, and for example if it were desired to use rubber lined headers on both sets of fluid. passages a second pair of headers identical in all respects to the rubber lined headers 32 and 42 could be fitted in place of the cast iron headers 20 and 26. This important feature of the design is. achieved by making all components asymmetric with respect to the central horizontal plane of the heat exchanger body. There is an even number of rows of fluid passages (for convenience a multiple of 4) for each of the fluids, spaced so that the central horizontal plane passes midway between a row of passages in one direction and the contiguous row of passages at right angles. The centre lines of the baiiles in the reversing headers 26 and 42 are respectively above and below the central plane by a distance equal to halfthe dis tance between the centres of contiguous rows of passages, and the baffles 01 the inlet and outlet headers 20 and 32- are asymmetric to an equal distance. Also the bolts joining the headers at the top and bottom are either both above or both below the bolts joining the headers for the other set of passages. Thus the pair of headers for one set of fluid passages is identical to the pair of headers for the other set of passages in respect of the spacing of the baffles, lugs and bolt holes when each of the headers in one pair is rotated in its own plane through an angle of 180.

We claim:

1. A method of making a heat exchanger of the type in which the heat exchanger body is of carbon and has first rows of parallel passages for one heat exchange fluid and adjacent second rows of parallel passages for another heat exchange fluid, the direction of the first rows crossing the direction of the second rows, which heat exchanger body comprises an assembly of a plumlity of contacting carbon plates and has said passag s p ovided by para lel. r ov s he p ates. said. method compr ng the steps oi c pin thev emble -bon plates; between two cover p ates at. a p essure not, substantially greater than that necessary to hold the carbon plates in position, placing the assembly in a cone tainer and evacuating the container, immersing the carbon plates Within the container in a resin in liquid state, applying pressure within the container thereby impregnating the carbon plates with, the resin, draining excess resin from the assembly, raising the pressure between the cover plates and the assembled carbon plates and thereafter curing the resin, thereby bonding the carbon plates together.

2. A method of making a heat exchanger of he. typ in w h t eheat exch ng r body is of carbon and has. first rows of parallel passages for one heat exchange fluid and adjacent second rows of parallel passages for another heat ex-. change. fluid, the direction of the first rows cross? ing the direction of the second, rows, which heat exchanger body comprises an assembly of a plu rality of contacting carbon plates and has said passages provided by parallel grooves in the plates, said method comprising the steps oi clamping the assembled carbon plates between two cover plates at a pressure not substantially higher than that necessary to hold the, carbon plates in position, placing the assembly in a container, subjecting the interior ofthe container to a vacuum of about 3 millimetres of mercury for about an hour, introducing into. the container a heat-polymerizable resin in liquid state and immersing the assembly therein, subjecting the interior of the container to a. pressure oi about pounds per square inch for about 2 hours, gradually releasing said pressure and allowing the assembly to drain, increasing the pressure between the cover plates and the assembled carbon plates to between 400 and 500 pounds per square inch and heating the assembly to cure the resin.

3. A method as claimed in claim 2 in which the resin is cashew nut shell resin, containing Polymerizatio catalyst.

4. A method as claimed in claim 3 in which the polymerization catalyst is diethyl sulphate and is present in an amount about 10% by weight of the resin.

5. A method as claimed in claim 3 in which the curing of the resin is effected by placing the assembly in an oven at about 80 C. for about 24 hours, during that period at intervals of about an hour removing the assembly from the oven and allowing it to cool before replacing it in the oven, thereafter heating the assembly at about C. for about 24 hours and finally heating the assembly at about C. for about 24 hours, thereafter allowing the body to cool.

6. A method as claimed in claim 2 in which the resin is the resin resulting from the reaction between sulphuric acid and a mix of furiural and furfuryl alcohol.

7. A method as claimed in claim 6 in which the resin is prepared by mixing equal parts by weight of furfural and furfuryl alcohol with between 0.1 and 0.2 percentage of normal aqueous solution of sulphuric acid, heating and cooling the mixture, neutralizing it with caustic soda, heating the mixture under reduced. pressure to remove water of condensation, and acidifying with a further quantity of between 0.1, and 9.2 percentage of normal aqueous soluti n of sub phuric acid. curing oi the re in eing effe ted y heating in an oven for 9 hours at about 80 C. and for a further 3 hours at about 150 C.

8. A method of making a heat exchanger of the type in which the heat exchanger body is of carbon and has first rows of parallel passages for one heat exchange fiuid and adjacent second rows of parallel passages for another heat exchange fiuid, the direction of the first rows crossing the direction of the second rows, which heat exchanger body comprises an assembly of a plurality of contacting carbon plates and has said passages provided by parallel grooves in the plates, said method comprising the steps of clamping the assembled carbon plates together at a pressure suflicient only to hold the plates in position, heating the assembly at about 120 C.

for about 3 hours, allowing it to cool to about 80 C., subjecting the assembly to a reduced pressure of about 3 millimetres of mercury for about an hour in a sealed container, introducing a resin in liquid form into the container and immersing the assembly therein, applying air pressure at about 100 pounds per square inch within the container for about two hours, releasing the air pressure over a period of about half an hour, draining ofi excess resin from the assembly, increasing the clamping pressure between the plates to between 400 and 500 pounds per square inch, and finally hardening the resin.

WILLIAM STEPHENS NORMAN.

CHARLES HENRY VICTOR SAWYER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,725,797 Harvey Aug. 27, 1929 1,834,895 Brossman Dec. 1, 1931 2,174,886 Kiefer Oct. 3, 1939 2,391,351 Schmidt Dec. 18, 1945 2,403,897 Aller July 16, 1946 2,471,600 Adams et a1 May 31, 1949 2,512,230 Greaves et al June 20, 1950 

1. A METHOD OF MAKING A HEAT EXCHANGER OF THE TYPE IN WHICH THE HEAT EXCHANGER BODY IS OF CARBON AND HAS FIRST ROWS OF PARALLEL PASSAGE FOR ONE HEAT EXCHANGE FLUID ADJACENT SECOND ROWS OF PARALLEL PASSAGES FOR ANOTHER HEAT EXCHANGE FLUID, THE DIRECTION OF THE FIRST ROWS CROSSING THE DIRECTION OF THE SECOND ROWS, WHICH HEAT EXCHANGER BODY COMPRISING AN ASSEMBLY OF A PLURALITY OF CONTACTING CARBON PLATES AND HAS SAID PASSAGES PROVIDED BY PARALLEL GROOVES IN THE PLATES, SAID METHOD COMPRISING THE STEPS OF CLAMPING THE ASSEMBLED CARBON PLATES BETWEEN TWO COVER PLATES AT A PRESSURE NOT SUBSTANTIALLY GREATER THAN THAT NECESSARY TO HOLD THE CARBON PLATES IN POSITION, PLACING THE ASSEMBLY IN A CONTAINER AND EVACUATING THE CONTAINER, IMMERSING THE CARBON PLATES WITHIN THE CONTAINER IN A RESIN IN LIQUID STATE, APPLYING PRESSURE WITHIN THE CONTAINER THEREBY IMPREGNATING THE CARBON PLATES WITH THE RESIN, DRAINING EXCESS RRESIN FROM THE ASSEMBLY, RAISING THE PRESSURE BETWEEN THE COVER PLATES AND THE ASSEMBLED CARBON PLATES AND THEREAFTER CURING THE RESIN, THEREBY BONDING THE CARBON PLATES TOGETHER. 